1. Field of the Invention
The present invention relates to a semiconductor light emitting device which is composed of a nitride based Group III-V compound semiconductor and produces radiant energy in the short wavelength region covering blue-violet radiation to ultraviolet radiation and a method for fabricating the same.
2. Description of the Related Art
Recently, with an increasing demand for blue-violet light emitting laser diodes which serve as next generation light sources for high-density optical disks, tremendous research effort has been being directed to development of gallium nitride (GaN) based Group III-V compound semiconductor light emitting devices which emit light in the short wavelength region covering blue-violet radiation to ultraviolet radiation. Since the optical disk systems are expected to perform higher-density and higher-speed recording as a recorder, there have arisen a need for GaN based semiconductor lasers having high optical power and high reliability.
As an attempt to increase the service life of a GaN based laser and thereby provide high reliability, there has been used a technique in which an insulating film made from silicon dioxide (SiO2) or the like is deposited on a GaN based semiconductor grown on a sapphire substrate and the GaN based semiconductors is selectively grown on this insulating film, thereby reducing the dislocation density. This selective growth is reported in “IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4 (1998) 483-489” (the first literature) and “Journal of Crystal Growth, Vol. 195 (1998) 328-332” (the second literature). According to the first literature, an SiO2 film is cyclically formed in a line and space pattern in the <1-100> direction of GaN so that the GaN based semiconductor layers selectively grown are planarized and linked together and the product thus formed can be used as a low dislocation density substrate. While direction indices and mirror indices are normally represented by putting a bar mark (−) on top of a figure, a minus symbol (−) is put, in this specification, before a figure (e.g., “−1” in the <1-100> direction). The second literature teaches that the growing speeds of the selectively grown GaN in a direction (hereinafter referred to as “substrate perpendicular direction”) perpendicular to the substrate and in a direction (hereinafter referred to as “substrate parallel direction”) parallel with the substrate are dependent of an open area ratio (Ws/(W1+Ws)) which is the ratio of an open area (width: Ws) to the sum of an area (width: W1) covered with the SiO2 film and the open area (width: Ws). Applications in which the technique of selective growth is used for a laser have been reported in “Applied Physics Letters, Vol. 77 (2000), 1931-1933” (the third literature) and “IEICE Transuction Electron, Vol. E83-C (2000) 529-535” (the fourth literature). According to the third literature, the dislocation density of a laser structure can be reduced from about 1E10 cm−2 to about 1E7 cm−2 by the selective growth. The fourth literature discusses that, by reducing dislocation density to the same degree as described earlier through selective growth with an open area ratio (Ws/W1+Ws) of 0.33 which corresponds to the case where the line width W1 of the SiO2 film is 8 μm and the width of the open area is 4 μm, the threshold current of the laser can be reduced and the service life can be increased in the order of 1,000 hours.
It is generally known that if a semiconductor laser is operated to output high optical power, there occurs Catastrophic Optical Damage (COD), that is, localized thermal destruction of the facets of a laser. Since many surface levels exist at the laser facets, many non-radiative recombinations occur through the surface levels. Therefore, injected carrier density decreases at the laser facets so that the laser facets become laser radiation absorbing regions and, in consequence, localized heating at the facets becomes significant. This is the phenomenon called “COD”. It has been thought that COD does not occur GaN based lasers, because the material, GaN is strong. However, the fourth literature “IEICE Transuction Electron, Vol. 77 Vol. E83-C (2000) 529-535” has reported by way of an actual example that if a GaN-based laser is operated to output an optical power as high as 30 mW, COD will occur, causing deterioration of the laser facets.
As measures to prevent COD, there have been employed a technique for facilitating dissipation of heat from a laser and a technique for providing the so-called window structure for laser facets. As the technique for facilitating dissipation of heat from a laser of these techniques, in the case of a semiconductor laser for outputting high optical power, packaging in which the semiconductor laser is mounted (p-side down) by means of a solder or sub-mount with the pn junction side thereof being close to the heat sink in order to effectively dissipate heat generated in high optical power operation toward the heat sink is effective. However, this packaging should be carefully done by keeping a space between the solder and the outputting facet in order to prevent the solder from sticking to the outputting facet. Therefore, even if p-side down packaging is carried out, heat dissipation slows down in the vicinity of the laser facet, increasing the likelihood of occurrence of COD.
On the other hand, the technique of providing the so-called window structure for laser facets is not generally employed in GaN based lasers, because the material of the lasers, GaN is too hard to make a window structure. A case where a window structure is provided for a GaN based laser is described in Japanese Patent Kokai Publication No. 2000-196188. In the current development of GaN based lasers intended for outputting of high optical power, attempts have been made to prevent occurrence of COD at laser facets by restraining overall heat generation in a laser through power consumption saving by a reduction in the operating current and operating voltage of the laser.
However, there is a limit in the restraint of overall heat generation in lasers for the next generation optical disk systems which seek higher density and higher speed recording, so that there still remains the problem of laser degradation due to occurrence of COD.